<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body{
            background-color: cadetblue;
        }
        div{
            width:200px;
            height:200px;
        /*    background-color: antiquewhite;*/
            /*如果背景是图片的话，透明效果比较明显*/
            background-color: rgba(0,0,0,0.2) ;

        }
    </style>
</head>
<body>
<div></div>
</body>
</html>